Thread: gear ratio

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Originally Posted by tudorkeith

hey guys, here's a quick little question for all you engineering types. in my rod, I have a 350/350 with a 2:76 rear. last year I rean a 25" tall tire, now I have a 30" tall bias ply WW. what will that do for performance , milage etc? what is the effective gear ratio? thanks

You can plug your numbers into this calculator and see what you get...http://www.ringpinion.com/Calc_RPM.aspx

The calculator Randyr linked you to uses a formula where Cruise RPM = (MPH x gear ratio x 336 x OD ratio) / Tire Diameter in inches. Plugging in your 2.76 rear gear ratio and no OverDrive (1.0 for top gear) your 25" tires pushed you to 2597rpm at 70mph, and your new 30" tires will drop that to 2164rpm with no other changes. The 336 is just a factor to align units and give you RPM.